Switch from x/net/context -> context

Since Go 1.7, context is a standard package. Since Go 1.9, everything
that is provided by "x/net/context" is a couple of type aliases to
types in "context".

Many vendored packages still use x/net/context, so vendor entry remains
for now.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

client/swarm_get_unlock_key.go

@@ -1,10 +1,10 @@
 package client // import "github.com/docker/docker/client"
 
 import (
+	"context"
 	"encoding/json"
 
 	"github.com/docker/docker/api/types"
-	"golang.org/x/net/context"
 )
 
 // SwarmGetUnlockKey retrieves the swarm's unlock key.